Effect of Sc and Zr alloying on microstructure and precipitation evolution of as cast Al-B4C metal matrix composites

Additions of Sc and Zr were introduced into Al-15 vol.-% B4C composites, and eight experimental composites with different Sc and Zr levels were prepared via a conventional cast process. Optical microscopy, SEM and TEM were applied for observing the as cast microstructures, including the interfaces between the Al matrix and the B4C as well as the evolution of the precipitates. It was found that Sc involved the interfacial reactions with B4C that partially consumed the Sc. On the other hand, no major Zr reaction products were found in the interfaces, and the major part of Zr remained in the matrix for precipitation strengthening. The Sc addition yielded considerable precipitation strengthening in the as cast and peak aged conditions. The combination of Sc and Zr significantly enhanced the precipitation strengthening. Nanoscale precipitates Al3Sc and Al-3(Sc, Zr) were found in the as cast microstructure and contributed to the significant increase of matrix hardness.